This invention relates to rheometers used for measuring fluid characteristics such as viscosity, which rheometers would but for the present invention tend to provide inaccurate measurements due to significant adverse end effects created by swirling fluid.
Various industries may need to test fluids to determine if they are suitable for their intended use. Fluid properties are often measured within pressurized environments. Fluid properties may be measured at elevated temperatures which may require pressure to prevent boiling. Certain fluids have a tendency to entrain air and fluid pressure is required to compress the air bubbles in the fluid. In real-time monitoring of processes, the fluid is continuously or semi-continuously pumped through the instrument at the process conditions.
Such fluids are often characterized as either Newtonian or non-Newtonian. To characterize a fluid as one of these, shear stress versus shear rate measurements are made. In Newtonian fluids, the shear stress versus shear rate is a constant called viscosity. Examples of Newtonian fluids are water and certain oils. In non-Newtonian fluids, the shear stress versus shear rate is not constant. Non-Newtonian fluids are classified by their shear stress versus shear rate curves as Power law, Bingham, or Pseudoplastic fluids. Examples of non-Newtonian fluids are gels, drilling muds, and cements. In non-Newtonian fluids, certain rheological properties or characteristics, such as n', K', yield stress, consistency, etc. are measured.
An apparatus used to measure shear stress versus shear rate is referred to herein as a rheometer, which term as used herein encompasses both multiple-speed testing and single-speed testing devices (the latter conventionally being referred to as a "viscometer" even if performed by the identical instrument capable of multiple-speed testing). In one embodiment, a rheometer is a couette type instrument in which a cup is turned at a constant speed or shear rate within a body of fluid contained in a chamber of a housing. A bob is suspended inside the cup. Fluid between the bob and cup imparts a torque or shear stress on the bob. This torque is measured and converted to the desired parameter viscosity) in a known manner. In an alternative embodiment, a paddle may be suspended inside the rotating cup. The resulting torque on the paddle can then be measured and converted to a fluid parameter (e.g., consistency).
In one such type of instrument, such as the Fann 35/50 series of rheometers, the sleeve of the rotating cup ends just after the bottom of the bob and the level of the fluid in the chamber does not extend much above the top of the bob. As a result, there is little swirl in the fluid at the respective ends of the bob. Because there is little swirl, the fluid imposes practically no effects on the ends of the bob to distort the measurement. Any such "end effects" which remain are essentially linear and can be readily compensated.
In a pressurized type of instrument, however, the entire system is full of fluid and in many such devices the bob is a thin walled tube because an extremely light part is needed. This common on devices such as the Brookfield TT-100 and the Brookfield TT-200. The combination of the device being filled with fluid along with the bob being basically a hollow tube makes the readings of this type of rheometer erratic and inconsistent. This because in this configuration the tested fluid undergoes moderate to very high swirl which creates moderate to very high non-linear end effects adversely affecting the rotation of the thin-wailed type bob, thereby affecting the measurements that are responsive to such rotation. Thus, there is the need for an improved instrument of this type which does not undergo such adverse end effects and which thus provides consistent, accurate measurements.